diff --git a/testing/web-platform/tests/mathml/presentation-markup/mrow/inferred-mrow-baseline.html b/testing/web-platform/tests/mathml/presentation-markup/mrow/inferred-mrow-baseline.html
index 1541b2d6ce17..c92cbba922f0 100644
--- a/testing/web-platform/tests/mathml/presentation-markup/mrow/inferred-mrow-baseline.html
+++ b/testing/web-platform/tests/mathml/presentation-markup/mrow/inferred-mrow-baseline.html
@@ -20,7 +20,7 @@
   window.addEventListener("load", runTests);
   function runTests()
   {
-      ["Mrow", "Sqrt", "Style", "Error", "Phantom", "Math", "Menclose", "Mpadded"].forEach((tag) => {
+      ["Mrow", "Sqrt", "Style", "Error", "Phantom", "Math", "Menclose", "Mpadded", "Unknown"].forEach((tag) => {
           var x = document.getElementById("above" + tag).getBoundingClientRect();
           var y = document.getElementById("below" + tag).getBoundingClientRect();
           test(function() {
@@ -43,8 +43,10 @@
     <math><mspace id="aboveMath" width="10px" height="30px" style="background: purple"></mspace><mspace id="belowMath" width="10px" depth="30px" style="background: blue"></mspace></math>
     <math><menclose notation="box"><mspace id="aboveMenclose" width="10px" height="30px" style="background: purple"
 ></mspace><mspace id="belowMenclose" width="10px" depth="30px" style="background: blue"></mspace></menclose></math>
-    <math><mpadded notation="box"><mspace id="aboveMpadded" width="10px" height="30px" style="background: purple"
+    <math><mpadded lspace="10px"><mspace id="aboveMpadded" width="10px" height="30px" style="background: purple"
 ></mspace><mspace id="belowMpadded" width="10px" depth="30px" style="background: blue"></mspace></mpadded></math>
+    <math><unknown><mspace id="aboveUnknown" width="10px" height="30px" style="background: purple"
+></mspace><mspace id="belowUnknown" width="10px" depth="30px" style="background: blue"></mspace></unknown></math>
   </p>
 </body>
 </html>
diff --git a/testing/web-platform/tests/mathml/presentation-markup/mrow/inferred-mrow-stretchy.html b/testing/web-platform/tests/mathml/presentation-markup/mrow/inferred-mrow-stretchy.html
index b6a51762dfda..4b89dd6fa5f0 100644
--- a/testing/web-platform/tests/mathml/presentation-markup/mrow/inferred-mrow-stretchy.html
+++ b/testing/web-platform/tests/mathml/presentation-markup/mrow/inferred-mrow-stretchy.html
@@ -31,7 +31,7 @@
   window.addEventListener("load", () => { document.fonts.ready.then(runTests); });
   function runTests()
   {
-      ["Mrow", "Sqrt", "Style", "Error", "Phantom", "Math", "Menclose", "Mpadded"].forEach((tag) => {
+      ["Mrow", "Sqrt", "Style", "Error", "Phantom", "Math", "Menclose", "Mpadded", "Unknown"].forEach((tag) => {
           var mo = document.getElementById("mo" + tag);
           test(function() {
               assert_true(MathMLFeatureDetection.has_mspace());
@@ -52,7 +52,8 @@
     <math><mphantom><mo style="visibilty: visible;" id="moPhantom">&#x21A8;</mo><mspace width="1px" height="100px" style="background: magenta"></mspace></mphantom></math>
     <math><mo id="moMath">&#x21A8;</mo><mspace width="1px" height="100px" style="background: magenta"></mspace></math>
     <math><menclose notation="box"><mo id="moMenclose">&#x21A8;</mo><mspace width="1px" height="100px" style="background: magenta"></mspace></menclose></math>
-    <math><mpadded notation="box"><mo id="moMpadded">&#x21A8;</mo><mspace width="1px" height="100px" style="background: magenta"></mspace></mpadded></math>
+    <math><mpadded lspace="10px"><mo id="moMpadded">&#x21A8;</mo><mspace width="1px" height="100px" style="background: magenta"></mspace></mpadded></math>
+    <math><unknown><mo id="moUnknown">&#x21A8;</mo><mspace width="1px" height="100px" style="background: magenta"></mspace></unknown></math>
   </p>
 </body>
 </html>
diff --git a/testing/web-platform/tests/mathml/presentation-markup/mrow/spacing.html b/testing/web-platform/tests/mathml/presentation-markup/mrow/spacing.html
new file mode 100644
index 000000000000..4bdfc9a001a3
--- /dev/null
+++ b/testing/web-platform/tests/mathml/presentation-markup/mrow/spacing.html
@@ -0,0 +1,51 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8">
+<title>Spacing in mrows</title>
+<link rel="help" href="https://mathml-refresh.github.io/mathml-core/#horizontally-group-sub-expressions-mrow">
+<link rel="help" href="https://mathml-refresh.github.io/mathml-core/#radicals-msqrt-mroot">
+<link rel="help" href="https://mathml-refresh.github.io/mathml-core/#style-change-mstyle">
+<link rel="help" href="https://mathml-refresh.github.io/mathml-core/#error-message-merror">
+<link rel="help" href="https://mathml-refresh.github.io/mathml-core/#making-sub-expressions-invisible-mphantom">
+<link rel="help" href="https://mathml-refresh.github.io/mathml-core/#the-top-level-math-element">
+<link rel="help" href="https://mathml-refresh.github.io/mathml-core/#enclose-expression-inside-notation-menclose">
+<link rel="help" href="https://mathml-refresh.github.io/mathml-core/#adjust-space-around-content-mpadded">
+<link rel="help" href="https://mathml-refresh.github.io/mathml-core/#operator-fence-separator-or-accent-mo">
+<meta name="assert" content="Spacing is added around operators inside mrow-like elements.">
+<script src="/mathml/support/feature-detection.js"></script>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script type="text/javascript">
+  setup({ explicit_done: true });
+  window.addEventListener("load", () => { document.fonts.ready.then(runTests); });
+  function runTests()
+  {
+      ["Mrow", "Sqrt", "Style", "Error", "Phantom", "Math", "Menclose", "Mpadded", "Unknown"].forEach((tag) => {
+          test(function() {
+              assert_true(MathMLFeatureDetection.has_operator_spacing());
+              var mrow = document.getElementById(tag);
+              var mn1 = mrow.firstElementChild.getBoundingClientRect();
+              var mn2 = mrow.lastElementChild.getBoundingClientRect();
+              assert_greater_than_equal(mn2.left - mn1.right, 50);
+          }, `operator spacing inside ${tag}`);
+      });
+      done();
+  }
+</script>
+</head>
+<body>
+  <div id="log"></div>
+  <p>
+    <math><mrow id="Mrow"><mn>1</mn><mo lspace="50px">|</mo><mn>2</mn></mrow></math>
+    <math><msqrt id="Sqrt"><mn>1</mn><mo lspace="50px">|</mo><mn>2</mn></msqrt></math>
+    <math><mstyle id="Style"><mn>1</mn><mo lspace="50px">|</mo><mn>2</mn></mstyle></math>
+    <math><merror id="Error"><mn>1</mn><mo lspace="50px"></mo><mn>2</mn></merror></math>
+    <math><mphantom id="Phantom"><mn>1</mn><mo lspace="50px">|</mo><mn>2</mn></mphantom></math>
+    <math id="Math"><mn>1</mn><mo lspace="50px">|</mo><mn>2</mn></math>
+    <math><menclose id="Menclose" notation="box"><mn>1</mn><mo lspace="50px">|</mo><mn>2</mn></menclose></math>
+    <math><mpadded id="Mpadded" lspace="10px"><mn>1</mn><mo lspace="50px">|</mo><mn>2</mn></mpadded></math>
+    <math><unknown id="Unknown"><mn>1</mn><mo lspace="50px">|</mo><mn>2</mn></unknown></math>
+  </p>
+</body>
+</html>